This should probably be as simple as using the same code as for JSOP_NEWARRAY. The documentation Opcodes.h has some comment about avoid unboxed arrays, but I don't think that really needs any special attention. Arai points out it's probably used here: https://dxr.mozilla.org/mozilla-central/rev/0eef1d5a39366059677c6d7944cfe8a97265a011/js/src/vm/ObjectGroup.cpp#1517. Unboxed-arrays are disabled by default.
This is a trivial part of bug 1338920.